1. Field of the Invention
The present invention relates to a light-emitting element driving circuit for use in driving a light-emitting element such as an LED.
2. Description of the Related Art
FIG. 12 shows an exemplary known light-emitting element driving circuit for driving semiconductor light-emitting elements such as an LED (JP-A 7-154015).
In this driving circuit, a driven LED 1 receives a drive current supplied from a constant current source 21. Between a cathode of the LED 1 serving as the light-emitting element and a ground terminal (GND), a switching element 22 and a resistor 23 are connected in this order to form a current path A. In parallel with the current path A, on the other hand, a resistor 24, a switching element 25 and a resistor 26 are connected serially to form a current path B. The switching element 25 is controlled on/off based on an input signal IN as a control signal. To the contrary, the switching element 22 is controlled on/off based on an inverted signal *IN of the input signal IN from an inverter 27. In a word, the switching elements 22 and 25 are differentially controlled on/off.
In this configuration, when the input signal IN is set “Low”, the switching element 22 is turned on and the switching element 25 is turned off. As a result, a drive current Id flows in the LED 1 and a current Ii flowing in the current path B turns to zero, resulting in emission of light from the LED 1. On the other hand, when the input signal IN is set “Hi”, the switching element 22 is turned off and the switching element 25 is turned on. As a result, the current Ii flows in the current path B and the drive current Id turns to zero, resulting in elimination of light from the LED 1.
The circuit of JP-A 7-154015 requires a semiconductor switching element each in the current path A containing the LED 1 as the light-emitting element and in the current path B as the switching unit.
FIG. 13 shows another example of the circuit for driving semiconductor light-emitting elements known in the art. In this case, an inverter 32 is employed as a driver element and its output terminal is connected to the anode (A) of the LED 1. A resister 33 is connected between the cathode (K) of the LED 1 and the ground terminal (GND). The resister 33 is employed to set a value of the forward current If flowing in the LED 1.
This driving circuit has an advantage over JP-A 7-154015 because of no requirement of switching elements though it has the following problem. Namely, control of elimination/emission of light from the LED 1 requires the anode-cathode potential to fluctuate between 0-2V. This fluctuation requires a certain time to charge and discharge a parasitic capacitance such as a junction capacitance of the LED 1 and causes a problem because the LED 1 can not be driven fast.
In consideration of this problem, a driving circuit has been proposed to suppress the fluctuation of the anode potential to drive the LED 1 quickly (see JP-A 12-232240, for example). FIG. 14 shows the driving circuit disclosed in this patent publication 2. In this case, the cathode (K) of the LED 1 is grounded. A current source 35 is connected between the supply voltage (Vcc) and the anode (A) of the LED 1. An inverter 34 is provided as a driver element to switch supply of current to the LED 1. The logic of the input signal IN fed from the input terminal of the inverter 34 is changed to switch the LED 1 on/off. Between the output terminal of the inverter 34 and the anode (A) of the LED 1, a diode 36 is connected in a forward direction directed from the anode (A) to the output terminal of the inverter 34.
In this driving circuit, for emission of light from the LED 1, the input signal IN is made “Low” to turn the output signal from the inverter 34 to “Hi”. If the supply voltage is equal to 5 V, the output signal from the inverter 34 is also equal to approximately 5 V and thus a reverse voltage is applied across the diode 36. Accordingly, the current is supplied from the current source 35 not to the diode 36 but to the LED 1, which emits light.
On the other hand, for elimination of light from the LED 1, the input signal IN is made “Hi” to turn the output signal from the inverter 34 to “Low”. In this case, a forward voltage is applied across the diode 36. Accordingly, the current is supplied from the current source 35 to the diode 36 and drained into the output terminal of the inverter 34. As a result, any current is not supplied to the LED 1, which turns off. While the LED 1 turns off, the anode potential of the LED 1 is kept equal to the forward voltage of the diode 36. For example, if the diode 36 consists of two serially connected diode elements each having a forward voltage of 0.8 V as shown in FIG. 14, the anode potential on the LED 1 comes to 2×0.8=1.6 V. Therefore, if the anode potential is equal to 2.0 V when the LED 1 turns on, for example, the anode potential on the LED 1 may fluctuate between 1.6–2.0 V. Accordingly, the fluctuation width can be reduced greatly over the driving circuit of FIG. 13. Thus, the time required to charge and discharge the LED 1 can be shortened to achieve high-speed driving.
In the driving circuit of FIG. 14, the anode potential on the LED 1 is controlled to have a small fluctuation width. Though, there is a problem because a large fluctuation of the cathode potential on the diode 36 causes the reverse voltage to exceed the breakdown voltage of the diode 36, resulting in device destruction possibly. Namely, in the driving circuit of FIG. 14, when the input signal IN to the inverter 34 is “Hi” and the output signal therefrom is “Low” (the LED 1 turns off in this case), the cathode potential on the diode 36 is almost equal to zero. On the other hand, the anode potential on the diode 36 is almost equal to the forward voltage of the diode 36, that is, 1.6 [V] (see FIG. 15). Therefore, no problem occurs because the forward voltage is applied across the diode 36. To the contrary, when the input signal IN to the inverter 34 is “Low” and the output signal therefrom is “Hi” (the LED 1 turns on in this case), the cathode potential on the diode 36 is almost equal to 5 [V]. On the other hand, the anode potential on the diode 36 is almost equal to the forward voltage of the LED 1, that is, 2 [V] (see FIG. 15). Therefore, a reverse voltage up to approximately 3 [V] is applied across the diode 36.
When such the driving circuit is fabricated in an integrated circuit, the diode 36 may be composed of a collector-base short-circuited bipolar npn transistor 36a. In structure, the diode 36 composed of the collector-base short-circuited bipolar npn transistor 36a has a higher band though it has a lower breakdown voltage against the reverse voltage, which is hardly elevated up to 3 [V]. The diode 36 may be composed of a base-emitter short-circuited bipolar npn transistor 36b to improve the breakdown voltage against the reverse voltage. Though, it has a problem because of a larger capacitance and a lower band. The diode 36 composed of the base-emitter short-circuited bipolar npn transistor 36b may cause disadvantageous leakage of current into the substrate depending on the structure.